dpkg-dev 1.15.3 introduced the following change:
* Support tags before symbols in symbols file contained in source
packages. The first two tags are "optional" (use it to not fail if
the symbol might disappear from the library without breaking the
ABI) and "arch" to restrict the set of architectures where the
symbol is supposed to exist. Thanks to Modestas Vainius
<modestas@vainius.eu> for the patch. See dpkg-gensymbols(1) for
more information. Closes: #521551
lintian should warn if symbol file contains tags but the package does
not build-depend on dpkg-dev (>= 1.15.3~).
The tricky part of implementing this is that currently all of Lintian's
analysis of symbol files is based on the binary package, and by that point
dpkg-gensymbols has already analyzed and removed the tags. Finding the
symbol files in the source package is a bit trickier since they can be
assembled on the fly if the package wishes to, and it would mean parsing
them again.